Weird voltages in T50

[DonMoose]

Picked up a T50 (and a conjoined-TL806 2x12 cab) at a massive 3block yard sale today.

The tag on the T50 said '$75 needs tube' and I figured I could at least use the iron and/or chassis for something if it's not repairable - got the head and cab for $60.

Neither channel works right or well, but I get something out of both. The reverb works gangbusters - much louder than either actual channel, so I walked V6 (reverb drive/recover) down the line to see if any of the other tubes were at fault - triode A of V4 being the main suspect. Also cleaned the FX return jack with a few pluggings-into. No joy. Sound fades in and out, is never very loud (except with reverb on) and both channels pretty distorted.

Anyway, I started looking at the voltages - B+, screen, preamp string and bias looked OK on the DC scale, but when I looked at the AC, I got readings of 800 to 1kV on the main filters and in the preamp. The input voltage is a nice 121VAC, so I'm reasonably sure it's not the meter.

I'm left with pilot error (all readings taken using same ground point, 600VAC scale on meter for AC - 200VAC gave out-of-range error, 600VDC scale for DC), or something BAD wrong in the rectifier/filter chain.

Ideas?

[Structo]

Whoa Nellie!

That's a heap of voltage!

Not sure what could cause that.
Is the PT a export or maybe a 100vac primary Japanese PT?

I wasn't sure what brand amp that was until I looked at your schematic.
I shows 500v for B+ but somehow that is doubled?
How is the rectifier and or filter caps?

Did you know Mike Soldano designed those?

Here is a article where he talks about them.

http://www.musicianshotline.com/archive ... /08_02.htm

[solderstain]

What do you get for AC voltage on the PT secondary with all the tubes removed?

[DonMoose]

With the tubes out, I get a nice, solid 385VAC at the rectifiers.
I get about in the neighborhood of 475VDC all down the B+ RC string.

I still get the 1085VAC across the filters, though.

[DonMoose]

More voltage info:

AC in: 123VAC
HV Secondary: 357VAC on stby, 354VAC on operate
After bridge: 490VDC, 1082VAC on stby, 473VDC, 1044VAC on operate
Bias: -50VDC, 0.1VAC operate or stby
Heaters: 6.3VAC
Switching voltage (HWRectified heaters) 3.3VDC, 6.7VAC at rectifier, same after RC filter.

Did ALL (but the bias) caps die at once?

[Wayne]

Some meters don't do well measuring AC when it's riding on DC - if you've got a 600V or bigger coupling cap, try putting it in series with your meter and measuring AC volts again.

Just a thought...

W

[DonMoose]

Some meters don't do well measuring AC when it's riding on DC - if you've got a 600V or bigger coupling cap, try putting it in series with your meter and measuring AC volts again.

Ah! That makes a lot more sense.

[Wayne]

Seems to me I had an analog meter that would read 2xDC if you connected it to an DC voltage with AC selected. If your meter is doing that, it would make your above situation make perfect sense.

W

[DonMoose]

Seems to me I had an analog meter that would read 2xDC if you connected it to an DC voltage with AC selected. If your meter is doing that, it would make your above situation make perfect sense.

The series cap did the trick - ripple measures (assuming a bunch of stuff) about 12VAC at the main filter and zero everywhere else.


Thanks, Wayne!

Found an other issue, starting an other thread.